ACCIDENT MODIFICATION FACTORS FOR USE IN THE PREDICTION OF SAFETY ON RURAL TWO-LANE HIGHWAYS

Roadway safety engineers and designers are often faced with decisions concerning the safety effects of highway treatments or design options and must have an accurate measure of a location's current safety to predict how alternative treatments will affect future safety. While highway engineers can utilize computer-aided design (CAD) software to obtain cost information, there is no computerized means of estimating safety benefits for new or rehabilitative highway projects. The Interactive Highway Safety Design Model (IHSDM), being developed by the U.S. Federal Highway Administration, will be an add-on module for CAD-based systems which will include a suite of evaluation tools for assessing the safety impacts of geometric design decisions. A key component of the IHSDM is a crash prediction algorithm which will estimate the number and severity of accidents on specified roadway segments. This algorithm is aimed at predicting the number and severity of crashes for alternative designs on 2-lane rural highways by combining historical crash data, regression model output, and accident modification factors (AMF) based on past research studies and expert judgment. This paper focuses on AMFs developed in this work for roadway segments and intersections. AMFs for 9 segment geometries and 7 intersection geometries are presented.